Oberfähnrich is the rank of a "pupil officer" (an Aspirant in the Armée de l'Air, or a Zászlós in Hungary if my table is right). There was no equivalent in VVS, RAF or USA but the rank immediatly above in Luftwaffe is Leutnant (= Leitenant (VVS) = 2nd Lieutenant (USAAF) = Pilot Officer (RAF)) and there are in VVS and USAAF ranks just under the above:
Mladshii Leitenant (VVS) and Flight Officer (USA).

But they are not equivalent of Oberfähnrich, because this German rank implies the idea of someone still "training" in an officer's school. It is possible that the USN rank of Ensign implies the same thing, and so will be a good traduction.

Laurent - 'Ensign' in USN is exactly the same as '2nd Lt' in USAAF or USMC.

Flight Officer (F/O) is Highest Enlisted rank in this case for the USAAF, but about the same as the current rank of Warrant Officer in the gray area between Officer and NCO's . The USMC actually permitted some NCO's at rank of Sargent in early days of WWII to pilot fighters but it was rare.

The USMC ranks and names for officers are identical to USAAF and USAF whereas the USN does it's 'own thing'